Data storage devices of the type known as “Winchester” disc drives are well known in the industry. These disc drives magnetically record digital data on several circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a spindle motor. In disc drives of the current generation, the discs are rotated at speeds of up to 15,000 revolutions per minute.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably positioned by an actuator assembly. Each head typically includes electromagnetic transducer read and write elements which are carried on an air bearing slider. The slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly each head in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the heads and the discs, the heads are attached to and supported by head suspensions or flexures. The data are written as tracks on the disc surface.
A closed loop servo system is typically used to control the position of the heads relative to the data tracks. The servo system generates a position error signal (PES) indicative of the position of the heads from servo information that is written to the discs during manufacturing of the disc drive. In response to the detected position, the servo system outputs current to an actuator motor (such as a voice coil motor, or VCM) used to pivot the actuator assembly, including the heads, over the disc surfaces.
It is an ever increasing trend in the industry to provide successive generations of disc drive products with ever increasing data storage capacities and data transfer rates. Because the disc surface area available for recording data remains substantially constant or even decrease as disc drive form factors become smaller, substantial advancement in areal recording densities, both in terms of number of bits that can be recorded on each track as well as the number of tracks on each disc, are continually being made in order to facilitate such increases in data capacity.
In the existing art, the servo information used to define the tracks has been written during disc drive manufacturing (after assembly of disc stack) using a highly precise servo track writer. While the tracks have been intended to be concentric, uncontrolled factors such as bearing tolerances, spindle resonance, and misalignment of the discs tend to introduce errors in the location of the servo information. Each track has been not perfectly concentric, but instead exhibits some random, repeatable variations. These variations are referred to as repeatable servo pattern runout (or RRO) and the RRO appears as a component of the PES. Variations that are not repeated are referred to as non-repeated servo pattern runout (NRRO).
RRO has become increasingly important as the track densities on a disc have increased. Ultimately, RRO can lead to an upper limit on the achievable track densities because the control of RRO consumes a part of a track alignment budget. Also, RRO reduces the range over which the servo system can provide stable servo control.
Recent advances in disc drive manufacturing have created a need for a disc drive with the servo information written, or prewritten, to the discs prior to assembly of the disc stack. In particular, some manufacturing efficiency has been realized by pre-writing the servo information to the discs during manufacturing of the discs themselves.
Thus, there is a need for a method for assembling the discs in a disc stack that minimizes servo pattern runout.